The present invention relates to a technique of aligning a mask pattern and a photosensitive substrate relative to each other, which is applicable to an exposure system used in a photolithography process in which a mask pattern is transferred onto a photosensitive substrate to produce semiconductor devices, for example. More particularly, the present invention relates to a technique of detecting a mask pattern provided on a photosensitive substrate.
In photolithography processes for producing, for example, semiconductor devices, liquid crystal display devices, thin-film magnetic heads, imaging devices (CCD), magneto-optical discs, etc., an exposure system is used in which an image of a transfer pattern formed on a photo-mask or reticle (hereinafter referred to collectively as xe2x80x9creticlexe2x80x9d) is transferred onto a photosensitive substrate, e.g., a wafer or glass plate coated with a photoresist, by a projection exposure method through a projection optical system, or by a proximity exposure method.
In such an exposure system, the reticle and the wafer must be aligned with respect to each other with high accuracy prior to the exposure process. In order to effect the alignment, the wafer has position detecting marks (alignment marks) formed (transferred by exposure) thereon in a previous processing step. By detecting the positions of the alignment marks, the position of the wafer (i.e., the circuit pattern on the wafer) can be accurately detected.
Examples of alignment mark detecting methods include a laser beam scan type detecting method, a laser interference type detecting method, etc., in which the position of an alignment mark is detected by detecting laser light scattered or diffracted by the mark. However, laser light has strong monochromatic properties; therefore, the use of laser light may cause position detection accuracy to be degraded on account of adverse effects such as multiple interference between the photoresist surface and the mark surface.
There is another position detecting method in which an alignment mark is illuminated by a broad-band light beam from a light source, e.g., a lamp, and the illuminated mark is imaged through an image-forming optical system to detect the position of the alignment mark on the basis of an image signal output from the image-forming optical system (this type of detection method will be hereinafter referred to as xe2x80x9cimaging type position detectionxe2x80x9d). In contrast to the above-described detection methods that use laser light, the imaging type position detection has a merit that the position detection is unlikely to be influenced by adverse effects such as the presence of the photoresist.
Under the present circumstances where the patterns of semiconductor integrated circuits and other similar devices become increasingly finer, a process of planarizing (i.e., leveling) the wafer surface has recently been introduced as a process carried out after a film deposition process and before a photolithography process. The planarization process has an effect of improving device characteristics by uniformizing the thickness of a deposited film from which a circuit pattern is to be formed, and also has an effect of improving the adverse effects on the wafer surface unevenness on the line width error of a transferred pattern during the photolithography process.
However, in a detection system where position detection is effected on the basis of a height difference between the recessed and projecting portions of an alignment mark or a reflectivity variation at the alignment mark portion on the wafer surface, the planarization process may make it impossible to detect an alignment mark because the height difference between the recessed and projecting portions of the alignment mark is considerably reduced by the planarization. Particularly, in a process for an opaque deposited film (i.e., a metal or semiconductor film), the alignment marks are covered with an opaque film of uniform reflectivity. Therefore, position detection depends only on the height difference between the recessed and projecting portions of the marks. Thus, planarization carried out for an opaque deposited film raises the most serious problem.
Incidentally, xe2x80x9cdark-field microscopesxe2x80x9d and xe2x80x9cphase-contrast microscopesxe2x80x9d are known as optical systems for detecting only a xe2x80x9cstepxe2x80x9d portion on a substantially flat object to be detected. One type of dark-field microscope is arranged such that an object to be detected is illuminated with a numerical aperture large enough to prevent direct light from entering the image-forming optical system, and an image is formed through the image-forming optical system by using only light scattered by the object. In another type of dark-field microscope, a circular light-blocking member is provided at a part of an optical Fourier transform plane (pupil plane) in the image-forming optical system with respect to the object to be detected such that the center of the circular light-blocking member coincides with the optical axis of the image-forming optical system, and an aperture stop (o stop) is provided by which the distribution of illuminating light over a plane in the illumination optical system which is conjugate with the light-blocking member, that is, an optical Fourier transform plane in the illumination optical system with respect to the object to be detected, is restricted within a circle which is in image-forming relation to the circular light-blocking member. As the aperture stop (o stop), a stop having an annular aperture may be employed.
A typical dark-field microscope is adapted to apply illuminating light to an object to be detected (e.g., a position detection mark on a wafer) and to form an image of only higher-order diffracted light (and scattered light), with zeroth-order diffracted light (regularly reflected light) being blocked, among light reflected and diffracted by the illuminated object. Among the diffracted light, zeroth-order diffracted light contains substantially no information concerning unevenness on the object or reflectivity variation of the object. However, higher-order (first- and higher-order) diffracted light contains such information. Therefore, the dark-field microscope enables steps to be made visible more clearly (with higher contrast) than the ordinary (bright-field) microscopes because the image is formed only from higher-order diffracted light with zeroth-order diffracted light being blocked.
Meanwhile, a typical phase-contrast microscope has a phase-contrast filter provided at a pupil plane of an image-forming optical system. The phase-contrast filter gives a phase difference between zeroth-order diffracted light and diffracted (and scattered) light of other orders. The quantity of higher-order (first- and higher-order) diffracted light generated from a mark pattern of low step is extremely small. However, in the phase-contrast microscope, zeroth-order diffracted light, which is generated in a large quantity, can also be contributed to image formation. Therefore, it is possible to obtain an image which is brighter (higher in light intensity) than in the case of the dark-field microscope. It should be noted that, when the intensity ratio of zeroth-order diffracted light to diffracted light of other orders is extremely high, the image contrast reduces, and therefore, zeroth-order diffracted light may be reduced under certain circumstances.
However, if the conventional dark-field microscope is used to detect a position detection mark on a wafer, not only zeroth-order diffracted light, which is not needed for image formation, but also diffracted light of relatively low order (i.e., useful diffracted light, which contributes to image formation) is blocked, causing the image contract and fidelity to be degraded.
In a case where a conventional phase-contrast microscope is used to detect a position detection mark on a wafer, not only zeroth-order diffracted light, which is not needed for image formation, but also diffracted light of other orders (i.e., useful diffracted light, which contributes to image formation) is subjected to phase-difference imparting action and light-reducing action, causing the image contrast and fidelity to be degraded.
In view of the above-described problems, an object of the present invention is to provide an position detecting apparatus which is capable of accurately and reliably detecting the position of a position detection mark, even a mark having an extremely small height difference (step) between recessed and projecting portions which constitute the mark.
The present invention is applied to an apparatus having: an illumination optical system for illuminating a position detection mark on a substrate by illuminating light (e.g., broad-band light or multi-wavelength light) in a predetermined wave band; an image-forming optical system for forming an image of the position detection mark on an imaging device by receiving light generated from the position detection mark; and an image processing system for calculating the position of the position detection mark on the basis of an image signal output from the imaging device. In the present invention, the apparatus is provided with an illuminating light beam limiting member for limiting the illuminating light beam at a first plane (pupil plane) in the illumination optical system which is practically in optical Foureir transform relation to the position detection mark so that the illuminating light beam passes through only a substantially annular first area on the first plane which is centered at an optical axis of the illumination optical system; and an image-forming light beam limiting member for substantially blocking the image-forming light beam distributed over a substantially annular second area on a second plane (pupil plane) in the image-forming optical system which second plane is practically in optical Foureir transform relation to the position detection mark, the second area being in image-forming relation to the first area.
In the present invention, the apparatus may be provided with the above-described illuminating light beam limiting member, and a light-block member for selectively blocking light distributed over the second plane in the image-forming optical system such that the light-blocking member substantially blocks light regularly reflected by a position detection mark having periodicity as it is illuminated with the illuminating light beam, but transmits only light reflected and diffracted by the mark. Alternatively, the apparatus may be provided with, in place of the above-described illuminating light beam limiting member, an optical member for controlling the intensity distribution of the illuminating light over the first plane in the illumination optical system such that the intensity of the illuminating light in an annular first area on the first plane is higher than in another area on the first plane, and the above-described image-forming light beam limiting member. The above-described illuminating light beam limiting member and image-forming light beam limiting member may be replaced by, respectively, a first stop member for passing an illuminating light beam distributed over an annular first area on a practical pupil plane of the illumination optical system, and a second stop member for passing an image-forming light beam distributed over an area other than an annular second area on a practical pupil plane of the image-forming optical system.
In the above-described arrangement, the substantially annular first area may be replaced by a first area having another predetermined shape, and the substantially annular second area may be replaced by a second area having a shape corresponding to the predetermined shape and being in image-forming relation to the first area.
It is desirable that the outer radius ro and inner radius ri of the annular first area should satisfy the following conditions:
rixe2x89xa7xcex2/(2xc3x97P) 
ro-rixe2x89xa6xcex1/P 
where xcex1 is the shortest wavelength in the wave band of light beams contributing to the formation of the image signal in the illuminating light, xcex2 is the longest wavelength in the wave band, and P is the period of the position detection mark.
It is desirable that the numerical aperture NAo of the image-forming optical system should satisfy the following condition:
NAoxe2x89xa7ro+xcex2/P 
It is preferable to provide a member for retaining the image-forming light beam limiting member (or the light-blocking member or the second stop member) in such a manner that the image-forming light beam limiting member can be selectively inserted into and withdrawn from the optical path of the image-forming optical system. In such a case, it is preferable to further provide a member for retaining the illuminating light beam limiting member (or the optical member or the first stop member) in such a manner that the illuminating light beam limiting member can be selectively inserted into and withdrawn from the optical path of the illumination optical system.
The apparatus may be further provided with an image forming device for forming an image of an index mark on the imaging device, so that a positional displacement between the image of the position detection mark and the image of the index mark is detected on the basis of an image signal output from the imaging device. It is desirable for the image forming device to have: an index board having the index mark; an illumination system for illuminating the index board by a light beam different from the illuminating light applied to the substrate surface; and an image-forming system for forming an image of the index mark on the imaging device by receiving light generated from the index mark. Particularly, the index board may be placed at a plane in the image-forming optical system which is practically conjugate to the substrate, so that the image-forming optical system forms an image of the position detection mark on the index board and also forms an image of the position detection mark and an image of the index mark on the imaging device.
The optical member for controlling the intensity distribution of the illuminating light over the first plane in the illumination optical system such that the intensity of the illuminating light in a first area having an annular or other predetermined shape on the first plane is higher than in another area on the first plane may be a stop member with a light-blocking portion which substantially covers the another area on the first plane so that the light intensity in the another area is substantially zero.
It is desirable for the optical member to have an intensity distribution changing member for change at least one of the outer and inner radii of the annular first area. The intensity distribution changing member may have a plurality of stop members which are different from each other in at least one of the outer and inner radii of an annular aperture, and a member for retaining the stop members such that one of the stop members is placed in the optical path of the illumination optical system. It is desirable for the image-forming light beam limiting member to change the radial width of a light-blocking portion which substantially covers the annular second area in accordance with a change of at least one of the outer and inner radii of the first area.
According to another aspect thereof, the present invention is applied to an apparatus having: an illumination optical system for illuminating a position detection mark on a substrate by illuminating light (e.g., board-band light or multi-wavelength light) in a predetermined wave band; an image-forming optical system for forming an image of the position detection mark on an imaging device by receiving light generated from the position detection mark; and an image processing system for calculating the position of the position detection mark on the basis of an image signal output from the imaging device.
In the present invention, the apparatus sis provided with an illuminating light beam limiting member for limiting the illuminating light beam at a first plane (pupil plane) in the illumination optical system which is practically in optical Foureir transform relation to the position detection mark so that the illuminating light beam passes through only a substantially annular first area on the first plane which is centered at an optical axis of the illumination optical system, and a phase-contrast member for giving a phase difference between an image-forming light beam distributed over a substantially annular second area on a second plane (pupil plane) in the image-forming optical system which second plane is practically in optical Fourier transform relation to the position detection mark, the second area being in image-forming relation to the first area, and an image-forming light beam distributed over an area on the second plane other than the second area.
Alternatively, the apparatus may be provided with an illuminating light beam limiting member for limiting the illuminating light beam at the first plane in the illumination optical system so that the illuminating light beam passes through only a substantially annular first area on the first plane which is centered at the optical axis of the illumination topical system, and a phase-contrast member for giving a phase difference to light distributed over the second plane in the image-forming optical system such that light regularly reflected by the position detection mark as illuminated by the illuminating light beam is different in phase from light other than the regularly reflected light.
The apparatus may be provided with an optical member for controlling the intensity distribution of the illuminating light or secondary light source (surface illuminant) over the first plane in the illumination optical system such that the intensity of the illuminating light in the substantially annular first area is higher than in another area on the first plane, and a phase-contrast member for giving a phase difference between the image-forming light beam distributed over a substantially annular second area on the second plane in the image-forming optical system, which second area is in image-forming relation to the first area, and the image-forming light beam distributed over an area on the second plane other than the second area.
The apparatus may be provided with a stop member for passing the illuminating light beam distributed over an annular first area on a practical pupil plane of the illumination optical system, and a phase-contrast member for giving a phase difference between the image-forming light beam distributed over a substantially annular second area on the second plane in the image-forming optical system, which second area is in image-forming relation to the first area, and the image-forming light beam distributed over an area other than the second area.
The apparatus may be provided with a member for forming a substantially annular secondary light source on a practical pupil plane of the illumination optical system, the secondary light source being centered at the optical axis of the illumination optical system (or a member for forming a plurality of light source images in a substantially annular area on the pupil plane which is centered at the optical axis), and a phase-contrast member for giving a phase difference between the image-forming light beam distributed over a substantially annular area on a practical pupil plane of the image-forming optical system, which substantially annular area is in image-forming relation to the secondary light source, and the image-forming light beam distributed over an area on the practical pupil plane other than the substantially annular area.
The apparatus may be provided with an optical member for controlling the light intensity distribution over a practical pupil plane of the illumination optical system such that the light intensity in a substantially annular area which is centered at the optical axis of the illumination optical system is higher than in an area on the practical pupil plane inside the substantially annular area, and a phase-contrast member for giving a phase difference between the image-forming light beam distributed over a substantially circular area on a practical pupil plane of the image-forming optical system, which substantially circular area is in image-forming relation to the inside area, and the image-forming light beam distributed over an area on the practical pupil plane other than the substantially circular area.
In the above-described arrangement, the substantially annular first area may be replaced by a first area having another predetermined shape, and the substantially annular second area may be replaced by a second area having a shape corresponding to the predetermined shape and being in image-forming relation to the first area.
It is desirable for the apparatus to have a member for reducing the image-forming light beam distributed over the annular area on the second plane in the image-forming optical system, that is, zeroth-order light distributed over the second plane. The light-reducing member may be integrally formed with the phase-contrast member, or it may be disposed in close proximity to the phase-contrast member or placed at a plane approximately conjugate to the phase-contrast member (i.e., pupil conjugate plane).
It is desirable for the phase-contrast member to give a phase difference of approximately (2m+1)xcfx80/2xc2x1xcfx80/4 [rad] (m is an integer) between the image-forming light beam distributed over the second area and the image-forming light beam distributed over the area other than the second area. In this case, it is possible to shift either the phase of the image-forming light beam distributed over the second area or the phase of the image-forming light beam distributed over the area other than the second area. It is also possible to shift both the light beams by different amounts so that the above-described phase difference is given between them.
It is desirable that the outer radius ro and inner radius ri of the annular first area should satisfy the following conditions:
rixe2x89xa7xcex2/(2xc3x97P) 
ro-rixe2x89xa6xcex1/P 
where xcex1 is the shortest wavelength in the wave band of light beams contributing to the formation of the image signal in the illuminating light, xcex2 is the longest wavelength in the wave band, and P is the period of the position detection mark.
It is desirable that the numerical aperture NAo of the image-forming optical system should satisfy the following condition:
NAoxe2x89xa7ro+xcex2/P 
It is preferable to provide a member for retaining the phase-contrast member in such a manner that the phase-contrast member can be selectively inserted into and withdrawn from the optical path of the image-forming optical system. In this case, it is preferable to further provide a member for retaining the illuminating light beam limiting member [or the optical member, the stop member or the secondary light source (light source image) forming member] in such a manner that the illuminating light beam limiting member can be selectively inserted into and withdrawn from the optical path of the illumination optical system.
The apparatus may be further provided with an image forming device for forming an image of an index mark on the imaging device, so that a positional displacement between the image of the position detection mark and the image of the index mark is detected on the basis of an image signal output from the imaging device. It is desirable for the image forming device to have: an index board having the index mark; an illumination system for illuminating the index board by a light beam different from the illuminating light applied to the substrate surface; and an image-forming system for forming an image of the index mark on the imaging device by receiving light generated from the index mark. Particularly, the index board may be placed at a plane in the image-forming optical system which is practically conjugate to the substrate, so that the image-forming optical system forms an image of the position detection mark on the index board and also forms an image of the position detection mark and an image of the index mark on the imaging device.
Further, it is desirable to provide a member for adjusting the intensity of the light beam illuminating the index mark in accordance with a change in light quantity of the image-forming light beam from the position detection mark which is incident on the imaging device due, for example, to insertion or withdrawal of the illuminating light beam limiting member (or the optical member, etc.) into or from the optical path of the illumination optical system or replacement of the illuminating light beam limiting member. For example, the intensity of the light beam is increased in response to the withdrawal of the illuminating light beam limiting member from the illumination optical path. The adjusting member may be adapted to change the electric power (current or voltage) supplied to a light source that emits the light beam. Alternatively, the adjusting member may comprise a plurality of light-reducing filters which are different in transmittance from each other, and which are interchangeably disposed in the beam path.
The optical member for controlling the intensity distribution of the illuminating light (or the secondary light source) over the first plane in the illumination optical system such that the intensity of the illuminating light in an annular first area on the first plane is higher than in another area on the first plant may be a stop member with a light-blocking portion which substantially covers the another area on the first plane so that the light intensity in the another area is substantially zero.
It is desirable for the optical member to have an intensity distribution changing member for changing at least one of the outer and inner radii of the annular first area. The intensity distribution changing member may have a plurality of stop members which are different from each other in at least one of the outer and inner radii of an annular aperture, and a member for retaining the stop members such that one of the stop members is placed in the optical path of the illumination optical system. It is desirable for the phase-contrast member to change at least one of the radial width and position of the annular second area in accordance with a change of at least one of the outer and inner radii of the first area.
The change of the outer and/or inner radius (i.e., the radial width and/or position) of the above-described annular first area (secondary light source) or the change of the intensity distribution of illuminating light can be realized by disposing an aperture stop which is formed, for example, by a liquid crystal device or an electrochromic device, at the illumination system pupil plane, or arranging a mechanism whereby one of a plurality of stop members which are different from each other in at least one of the outer and inner radii of the aperture portion can be interchangeably disposed in the optical path. Alternatively, the arrangement may be such that a variable aperture stop is placed at the pupil plane so that the aperture diameter can be changed as desired (alternatively, a plurality of aperture stops having different aperture diameters are arranged to be capable of being interchangeably disposed in the optical path), thereby changing the outer radius of the first area. Further, a plurality of circular light-blocking plates having different diameters are arranged to be capable of interchangeably disposed in the optical path, thereby changing the inner radius of the first area.
The change of the outer and/or inner radius (i.e., the radial position and/or width) of the above-described annular second area can be realized, for example, by arranging a mechanism whereby one of a plurality of transparent substrates which are different from each other in at least one of the radial position and width of an annular phase shifter (dielectric film or the like) or an annular recess (or projection) can be interchangeably disposed in the optical path. It should be noted that a phase shifter may be provided in an area other than the annular second area instead of providing the annular phase shifter. Alternatively, the change of the outer and/or inner radius of the second area may be realized simply by arranging a plurality of circular transparent plates which are approximately equal in optical thickness but different in diameter from each other such that one of these plates can be interchangeably disposed in the optical path. It should, however, be noted that when the outer radius of the above-described first area is changed, it is impossible to give a phase difference between the image-forming light beam in the second area, which is an image-forming relation to the changed first area, and the image-forming light beam outside the second area; however, no problem will arise unless the image contrast or fidelity is degraded to such an extent that the desired position detection accuracy cannot be obtained by the change in the outer radius of the first area. It should be noted that, if the degradation of the image contrast or fidelity causes a problem, the image-forming light beam distributed outside the second area should preferably be block by a variable aperture stop, for example.
In the present invention, it is noted that a position detection mark on a substrate, e.g., a wafer, generally has a predetermined periodicity (period P) in the direction for position detection. In order that diffracted light, exclusive of zeroth-order diffracted light (regularly reflected by the mark), generated by the periodicity of the mark should contribute efficiently to the formation of a mark image, stops (light-blocking portions) for realizing dark-field conditions are respectively set for the secondary light source of the illumination optical system [i.e., the illuminating light beam distribution or illuminating light intensity distribution at a Fourier transform plane (pupil plane) in the illumination optical system] and at a Foureir transform plane (pupil plane) in the image-forming optical system, that is, at a plane conjugate to the pupil plane of the illumination optical system). Consequently, it is possible to efficiently utilize useful diffracted light while effectively blocking unwanted zeroth-order diffracted light.
That is, in the present invention, an illuminating light beam (secondary light source) at a first plane (pupil plane) in the illumination optical system which is practically in optical Foureir transform relation to a position detection mark is restricted within a substantially annular first area which is centered at an optical axis of the illumination optical system, and an image-forming light beam is substantially blocked which is distributed over a substantially annular second area, which is in image-forming relation to the first area, on a second plane (pupil plane) in the image-forming optical system which is in practically optical Foureir transform relation to the position detection mark. In other words, zeroth-order light from the position detection mark which is distributed over the second plane is substantially blocked. Alternatively, the illuminating light intensity distribution over the first plane in the illumination optical system is made high in the annular first area than in another area on the first plane, and the image-forming light beam distributed over the substantially annular second area, which is in image-forming relation to the first area, on the second plane in the image-forming optical system is substantially blocked. The arrangement may be such that the illuminating light beam is allowed to pass through only an annular first area on a practical pupil plane of the illumination optical system, and the image-forming light beam is allowed to pass through only an area other than an annular second area on a practical pupil plane of the image-forming optical system.
Accordingly, it becomes possible to detect the position of a position detection mark reliably (with an image of high contrast, even a position detection mark having an extremely small height difference (step) between the recessed and projecting portions of the mark. It should be noted that the annular second area on the second plane in the image-forming optical system need not completely be shielded from light, and that the annular second area may be given a predetermined transmittance, that is, the annular second area may be formed as a light-reducing portion, provided that the desired position detection accuracy can be obtained even if the image contrast or fidelity is slightly degraded. Although the above-described first and second areas have annular shapes, these area may have another shape, for example, a rectangular, square or polygonal (particularly, regular polygonal) shape. Further, the first area on the first plane (pupil plane) in the illumination optical system may be partially shielded (for formed as a light-reducing portion). That is, the first area may comprise a plurality of partial area (having any desired shape, e.g., a circular-arc shape, a circular shape, or a straight-line shape). Correspondingly, the second area on the second plane (pupil plane( in the image-forming optical system may be formed in the same shape as the first area. Alternatively, the second area may be formed in the shape of an annular ring, a rectangle, a polygon, etc. substantially containing a plurality of partial areas which are in image-forming relation to the first area.
Further, the outer radius ro and inner radius ri of the annular first area are set so as to satisfy the following conditions:
rixe2x89xa7xcex2/(2xc3x97P) 
ro-rixe2x89xa6xcex1/P 
were xcex1 is the shortest wavelength in the wave band of light beams contributing to the formation of the image signal in the illuminating light, xcex2 is the longest wavelength in the wave band, and P is the period of the position detection mark.
In addition, the numerical aperture NAo of the image-forming optical system is set so as to satisfy the following condition:
NAoxe2x89xa7ro+xcex2/P 
Accordingly, an image of a position detection mark of low step can be detected with high contrast. Further, it is preferable to provide a variable aperture stop (NA stop) for changing the numerical aperture NAo of the image-forming optical system at the second plane (pupil plane) in the image-forming optical system or at a plane conjugate to it such that the variable aperture stop will not mechanically interfere with the image-forming light beam limiting member (or the light-blocking member or the second stop member). Thus, even when the period of the position detection mark to be detected changes, the numerical aperture of the image-forming optical system can be set at a value corresponding to the new period so as to satisfy the above-described condition. Accordingly, the mark image can be detected with high contract at all times. It should be noted that the variable aperture stop may be displaced in the optical axis direction relative to the pupil plane of the image-forming optical system or the plane conjugate to it.
Further, the apparatus is provided with a member for retaining the image-forming light beam limiting member (or the light-blocking member of the second stop member) in such a manner that the image-forming light beam limiting member can be selectively inserted into and withdrawn from the optical path of the image-forming optical system. Accordingly, bright-field detection and dark-field detection can be switched over from one to the other, and thus it is possible to select either of the two different detection modes accordingly to the step height of the position detection mark to be detected. Therefore, a mark image of high contrast can be obtained at all times independently of the step height of the position detection mark, and the position detection accuracy can be improved. The apparatus is also provided with a member for retaining the illuminating light beam limiting member (or the optical member or the first stop member) in such a manner that the illuminating light beam limiting member can be selectively inserted into and withdrawn from the optical path of the illumination optical system. Accordingly, annular zone illumination and ordinary illumination can be switched over from one to the other, and for a position detection mark of relatively high step, even if the reflectivity of the mark is low, the mark image can be reliably detected by the ordinary illumination.
The optical member for controlling the intensity distribution of the illuminating light over the first plane in the illumination optical system such that the intensity of the illuminating light in an annular first area on the first plane is higher than in another area on the first plane has an intensity distribution changing member for changing at least one of the outer and inner radii of the annular first area. Therefore, even when the period of the position detection mark to be detected changes, at least one of the outer and inner radii of the annular first area can be set at a value corresponding to the new period so as to satisfy the above-described condition. Accordingly, it is possible to obtain a mark image of high contrast at all times independently of the period of the position detection mark. It should be noted that the outer and inner radii of the annular first area need not be changed in accordance with a change of the period of the position detection mark to be detected, and that the outer and/or inner radius of the annular first area may be changed only when the image contrast or fidelity is degraded by a change of the period of the position detection mark to such an extent that the desired position detection accuracy cannot be obtained.
Further, the image-forming light beam limiting member changes at least one of the radial width and position of a light-blocking portion which substantially covers the annular second area in accordance with a change of at least one of the outer and inner radii of the first area. Accordingly, even when at least one of the outer and inner radii of the annular first area changes in accordance with the period of the position detection mark to be detected, it is possible to block unwanted zeroth-order diffracted light and to enable useful diffracted light other than the zeroth-order diffracted light, which contributes to the image formation, to enter the imaging device efficiently at all times. It should be noted that the width and position of the light-blocking portion of the image-forming optical system need not be changed in accordance with a change of the outer and/or inner radius of the annular first area (i.e., a change of the period of the position detection mark to be detected), and that the masking width and/or position of the light-blocking portion may be changed only when the image contrast or fidelity is degraded by a change of the outer and/or inner radius of the annular first area to such an extent that the desired position detection accuracy cannot be obtained.
It should be noted that the change of the outer and/or inner radius (i.e., the radial width and/or position) of the above-described annular first and second areas can be realized by disposing an aperture stop which is formed, for example, by a liquid crystal device or an electrochromic device, at each pupil plane. Alternatively, in the case of the illumination optical system the change of the outer and/or inner radius of the annular first area can be realized by arranging a mechanism whereby one of a plurality of stop members which are different from each other in at least one of the outer and inner radii of the aperture portion can be interchangeably disposed in the optical path of the illumination optical system; in the case of the image-forming optical system, the change of the outer and/or inner radius of the annular second area can be realized by arranging a mechanism whereby a plurality of stop members which are different from each other in at least one of the outer and inner radii of the light-blocking portion can be interchangeably disposed in the optical path of the image-forming optical system.
According to the second aspect of the present invention, the shape of a phase-contrast member and the shape of a secondary light source of the illumination optical system (i.e., the illuminating light beam distribution or the illuminating light intensity distribution over the pupil plane of the illumination optical system) are set so as to minimize the effect of the phase-contrast member on diffracted light, exclusive of zeroth-order diffracted light, generated by the periodicity of the position detection mark on a substance, e.g., a wafer. Accordingly, a phase difference can be preponderantly given to only the zeroth-order diffracted light, and it is possible to reduce the intensity of the zeroth-order diffracted light.
That is, in the present invention, the illuminating light beam at a first plane (pupil plane) in the illumination optical system which is practically in optical Fourier transform relation to the position detection mark is restricted within a substantially annular first area which is centered at an optical axis of the illumination optical system, and a phase difference is given between the image-forming light beam distributed over a substantially annular second area, which is in image-forming relation to the first area, on a second plane (pupil plane) in the image-forming optical system which is in practically optical Fourier transform relation to the position detection mark, and the image-forming light beam distributed over an area on the second plane other than the second area. In other words, a phase difference is given between zeroth-order light from the position detection mark which is distributed over the second plane and light other than the zeroth-order light.
The arrangement may be such that the intensity of distribution of the illuminating light (or secondary light source or surface illuminant) over the first plane in the illumination optical system is made higher in the annular first area than in another area, or that the illuminating light beam is allowed to pass through only the annular first area on the practical pupil plane of the illumination optical system, or that a substantially annular secondary light source (surface illuminant) which is centered at the optical axis of the illumination optical system is formed on the practical pupil plane of the illumination optical system. It is also possible to form a plurality of light source images in a substantially annular area centered at the optical axis of the illumination optical system. The arrangement may also be such that the light intensity distribution over the practical pupil plane of the illumination optical system is made higher in a substantially annular area centered at the optical axis of the illumination optical system that in an area inside the substantially annular area, and that a phase difference is given between the image-forming light beam distributed over a substantially circular area, which is in image-forming relation to the inside area, on the practical pupil plane of the image-forming optical system, and the image-forming light beam distributed over an area other than the substantially circular area.
Accordingly, it becomes possible to detect the position of a position detection mark reliably (with an image of high contrast), even a position detection mark having an extremely small height difference (step) between the recessed and projecting portions of the mark. Although the above-described first and second areas and secondary light source (surface illuminant) have annular shapes, these areas may have another shape, for example, a rectangular, square or polygonal (particularly, regular polygonal) shape. Further, the first area on the first plane (pupil plane) in the illumination system pupil plane may be partially shielded (or formed as a light-reducing portion). That is, the first area may comprise a plurality of partial areas (having any desired shape, e.g., a circular-arc shape, a circular shape, or a straight-line shape). Correspondingly, the second area on the second plane (pupil plane) in the image-forming optical system may be formed in the same shape as the first area. Alternatively, the second area may be formed in the shape of an annular ring, a rectangle, a polygon, etc. substantially containing a plurality of partial areas which are in image-forming relation to the first area.
Further, the apparatus is provided with a member for reducing the image-forming light beam distributed over the annular area on the second plane in the image-forming optical system, that is, zeroth-order diffracted light distributed over the second plane. By doing so, it is possible to reduce the intensity ratio of zeroth-order diffracted light to diffracted light of other orders from the position detection mark, and hence possible to detect the position detection mark image with higher contrast.
Further, the outer radius ro and inner radius ri of the annular first area are set so as to satisfy the following conditions:
rixe2x89xa7xcex2/(2xc3x97P) 
roxe2x88x92rixe2x89xa6xcex1/P 
where xcex1 is the shortest wavelength in the wave band of light beams contributing to the formation of the image signal in the illuminating light, xcex2 is the longest wavelength in the wave band, and P is the period of the position detection mark.
In addition, the numerical aperture NAo of the image-forming optical system is set so as to satisfy the following condition:
NAoxe2x89xa7ro+xcex2/P 
Accordingly, an image of a position detection mark of low step can be detected with high contrast.
Further, it is preferable to provide a variable aperture stop (NA stop) for changing the numerical aperture NAo of the image-forming optical system at the second plane (pupil plane) in the image-forming optical system or at a plane conjugate to it such that the variable aperture stop will not mechanically interfere with the phase-contrast member. Thus, even when the period of the position detection mark to be detected changes, the numerical aperture of the image-forming optical system can be set at a value corresponding to the new period so as to satisfy the above-described condition. Accordingly, the mark image can be detected with high contrast at all times. It should be noted that the variable aperture stop may be displaced in the optical axis direction relative to the pupil plane of the image-forming optical system or the plane conjugate to it.
Further, the apparatus is provided with a member for retaining the phase-contrast member in such a member that the phase-contrast member can be selectively inserted into and withdrawn from the optical path of the image-forming optical system. Accordingly, phase-contrast detection and bright-field detection can be switched over from one to the other, and thus it is possible to select either of the two different detection modes, that is, to determine whether to use the phase-contrast member, according to the step height of the position detection mark to be detected. Therefore, a mark image of high contrast can be obtained at all times independently of the step height of the position detection mark, and the position detection accuracy can be improved.
The apparatus is also provided with a member for retaining the illuminating light beam limiting member (or the optical member or the first stop member) in such a manner that the illuminating light beam limiting member can be selectively inserted into the and withdrawn from the optical path of the illumination optical system. Accordingly, annular zone illumination and ordinary illumination can be switched over from one to the other, and for a position detection mark of relatively high step, even if the reflectivity of the mark is low, the mark image can be reliably detected by the ordinary illumination.
The optical member for controlling the intensity distribution of the illuminating light (or secondary light source of surface illuminant) over the first plane in the illumination optical system such that the intensity of the illuminating light in an annular first area on the first plane is higher than in another area on the first plane has an intensity distribution changing member for changing at least one of the outer and inner radii of the annular first area. Therefore, even when the period of the position detection mark to be detected changes, at least one of the outer and inner radii of the annular first area can be set at a value corresponding to the new period so as to satisfy the above-described condition. Accordingly, it is possible to obtain a mark image of high contrast at all times independently of the period of the position detection mark. It should be noted that the outer and inner radii of the annular first area need not be changed in accordance with a change of the period of the position detection mark, and that the outer and/or inner radius of the annular first area may be changed only when the image contrast or fidelity is degraded by a change of the period of the position detection mark to such an extent that the desired position detection accuracy cannot be obtained.
Further, the phase-contrast member changes at least one of the radial width and position of the annular second area in accordance with a change of at least one of the outer and inner radii of the first area. Accordingly, even when at least one of the outer and inner radii of the annular first area changes in accordance with the period of the position detection mark to be detected, it is possible to enable diffracted light to enter the imaging device while giving a phase difference to only the zeroth-order diffracted light at all times. It should be noted that the width and position of the annular second area of the image-forming optical system need not be changed in accordance with a change of the outer and/or inner radius of the annular first area (i.e., a change of the period of the position detection mark to be detected), and that the width and/or position of the phase shift portion may be changed only when the image contrast or fidelity is degraded by a change of the outer and/or inner radius of the annular first area to such an extent that the desired position detection accuracy cannot be obtained. Further, it is not necessary to change both the outer and inner radii of the second area in accordance with a change of the outer and inner radii of the first area; it suffices to change only the inner radius, for example, of the second area.